1. Field of the Invention
The present invention relates to seismic survey equipment. In particular, the invention relates to equipment assembly combinations and the logistics of equipment deployment.
2. Description of the Related Art
In principle, a seismic survey represents an analysis of the earth's geologic structure as indicated by seismic reflections from impedance discontinuities at lithologic interfaces. The analysis is influenced by seismic wave propagation velocities respective to the successively deeper geologic formations. A precisely-timed seismic source event, such as the ignition of buried explosives in a shallow borehole or a controlled mechanically-induced continuous vibration is launched at a precisely known location and time. Seismic wave reflections from this man-made seismic event are detected by a multiplicity of geophone or hydrophone sensor arrays located in a more-or-less orderly grid over the area of interest. A series of such seismic source events is initiated over the area of interest. The positions of the sensor arrays may be shifted to better receive the seismic reflections of interest prior to each successive seismic source event. The location of each sensor array and each source event is precisely mapped.
As a seismic wave from the timed event travels out from the source, reflections from that original seismic wave return to the surface where they are detected by the sensor arrays. The sensor arrays respond to the receipt of a wave with a corresponding analog electrical signal. These analog signals are received by data acquisition modules that digitize the analog signal stream for retransmission to a central recording unit. Among the significant data digitized by data acquisition modules is the amplitude or the strength of the reflected wave and the time lapse between the moment the event occurred and the moment the amplitude of the wave is received. For each seismic source event and each sensor array, amplitudesf are sampled over a time range typically from zero to five seconds, for an impulsive source such as the buried explosive; or zero to twenty seconds for the continuous vibratory source, for example. Samples are typically repeated every 2 milliseconds, thus generating from two to ten thousand samples per seismic source event per source array in representative cases for impulsive and vibratory sources.
In a single survey, there may be thousands of seismic source events each with thousands of seismic sensor arrays. Consequently, the data flow must be orderly and organized. For example, the data acquisition modules transmit digital sensor signal values in digital data packages containing a predetermined number of digital data bits. Each of these data packages may carry the identity of the specific sensor array from which the data originates and the time it was received by the sensor array in addition to the seismic signal amplitude value. The acquisition modules are programmed to transmit data packets respective to each sensor channel at a predetermined frequency. The variable data in a data packet represents an instantaneous snapshot of the analog signal flow from the sensor array channel. There may be numerous individual sensor arrays transmitting respective analog signals to the data acquisition module on the same communication channel.
Managing an orderly flow of this massive quantity of data to a central recording unit requires a plurality of geographically-distributed digital signal processing devices. The data acquisition modules convert the sensor array analog data to digital data and transmit the digital data packets along receiver lines or radio transmission channels. There may be numerous data acquisition modules transmitting data packets along a single receiver line or channel. Among the functions of each data acquisition module is data packet transmission timing respective to the flow of data packets from other data acquisition modules transmitting respective data packets along the same receiver line. Typically, two or more receiver lines connect with base line units that further coordinate the data packet flow of numerous additional base line units into a base transmission line for receipt by a central recording unit.
Seismic surveying is often carried out under extremely inhospitable conditions of heat or cold, tropics or arctic, land and sea, desert or swamp. The equipment must be robust and extremely reliable so that it may withstand the conditions imposed by the natural physical environment. It must be also be able to survive and continue to function during frequent episodes of deployment, pick-up, transportation and redeployment.
It has been the practice in the seismic industry to build special purpose adaptations for equipment suitable for a certain type of physical environment. When a seismic survey requires sensor arrays to be placed on the bottom of a body of water it may also be desirable for reasons of operational efficiency to place the seismic data acquisition modules in proximity to the arrays at the water bottom. Resistance to invasion by water by the modules and the connectors that join the cables to the modules is essential for successful operation in this sub-aqueous environment. Specially designed module packaging and cable connectors are widely used for placement at water depth in excess of a few meters. Occasionally, in spite of these efforts, the cable connector fails when the module is submerged, resulting in flooding of the internal chamber and destruction the essential electronic functionality.
In contrast, seismic surveys in dry environments may have no requirement for placement of modules and their connectors under water. A less pressure-resistant module packaging and type of cable connector is less costly to build and could be perfectly robust in this dryer physical environment. Therefore it is common practice to use different types of module packages and cable connectors in dry land operations as compared to those used in water bottom environments. Similarly, adaptations are made for other differing environments such as swamp, arctic, jungle, urban etc.
Another category of reasons for selecting different types of cables and connectors for different seismic projects relates to the need to modify the type of cable and number of conductors to meet the geophysical or economic objectives of the survey. Variable numbers of channels may be accommodated by the modules (from 1 to 8, e.g.) and use of this feature allows the operator to optimize the equipment configuration for different types of surveys, but full optimization may necessitate use of a different type of cable connector (and cable).
Cable connectors are integrated into the module packaging and may be replaced as required, either for reasons of equipment modification to meet survey requirements, or to replace faulty connectors. Module packages that have been available in the industry do require opening of the chamber containing the electronic assemblages in order for the cable connectors to be replaced. This is laborious and subjects the electronic assemblages to risk of physical damage and to risk of invasion by contaminants, potentially causing equipment failure. It would be desirable to be able to replace the cable connectors of the module package without having to expend labor to open the electronics chamber or to risk equipment failure.
For module packages that have been available in the industry, failure of cable connectors can cause invasion of modules by water and/or other contaminants, rendering the electronics inoperable. Electronics must be replaced in most such cases causing labor and spare parts costs to escalate as well as causing lost production time. Failure may be caused by high external pressure during submergence or it may have other causes, often relating to the physical impacts incurred in the frequent episodes of field deployment, transportation and re-deployment. It would be highly desirable to have a novel module design in which failure of the cable connector does not cause damage to the internal electronics.
Seismic surveys normally require a finite period of field activity for completion, ranging from a few days to a few months. The practitioner, usually a seismic contracting company, desires to use the same data acquisition equipment on each successive project to minimize his costs and maximize his profitability. However, because often the successive survey projects may be in widely differing physical environments and may have distinctly different geophysical requirements, he has been required to maintain multiple types of data acquisition modules and multiple types of cables and connectors, suffering idle capacity and capital value when the specialized equipment is not required.
Data acquisition modules of many prior art designs and types are available in the industry. However, these prior art designs have been known to suffer from such problems as cable connector failure, flooding of interior electronics when cable connectors physically fail, and inability to operate under the entire range of physical environments without replacement of the exterior housing of the module. The unique features of the present invention, are proposed to overcome these limitations.
It would thus be desirable to have a module package that could be used universally in all types of environments without the need to make modifications related to the environment of utilization. If this type of packaging were available the operator could avoid cost of modifying the modules or replacing modules when going from a water bottom survey to a desert survey, for example. Or he could avoid the cost of maintaining two complete sets of modules both of which would be underutilized.
The adaptation of the universal seismic data acquisition module would merely require the replacement of the cable connectors to prepare it for the next seismic project (in a different environment or with a different cable type from the prior project). Thus the cost of equipment inventory and labor to effect change of cable connectors would be favorably impacted, as would the amount of time required to mobilize for the subsequent project.
Such a seismic data acquisition module, adaptable to all physical working environments, with the capabilities desired for easy and safe change of cable connectors and for protection of the electronics in event of connector failure has been invented and is described in the subsequent sections of this disclosure.